Filtering apparatus or filters usually used for removing material contained in suspension in fluids to be treated, such as untreated water, are formed by beds of granular materials, of the sand kind, anthracite, pumice stone, biolite, etc.. These materials have a characteristic weight per unit of volume greater than that of the fluid to be treated and form a bed called "fixed bed" resting on a support called floor of the filter. During the filtering phase, the fluid flows in a downward flow into this bed which remains fixed. In the rest of the text, when mention is made of mass per unit volume, the characteristic mass per unit volume will be meant. During this phase, the materials in suspension retained by this bed clog up this latter in depth.
Washing of the filtering bed at regular intervals removes the material retained in the two following steps. Such washing on the one hand causes detachment of the retained materials and, on the other, transport of these materials to a discharge point. Generally the second step is provided by means of water.
The washing phase is carried out, either with a so called fluidized bed or with a so called agitated bed. Washing by means of a fluidized bed consists in washing the filtration bed by return of water in an upward flow, at a speed higher than the minimum fluidization speed. In this case, the fixed bed in which the filtering materials were in contact with each other expands and the grains of materials which are held in suspension in the fluid stream no longer touch each other. Consequently, during this phase, the materials in suspension are no longer retained by the grains of the filtering material and become detached. Washing by means of an agitated bed consists in causing water to flow upwardly at a speed less than the minimum fluidization speed and simultaneously introducing air into this upward flow. The introduction of air bubbles causes both lowering of the mean density of the fluid and agitation of the filtering materials. During such agitation, the materials in suspension become detached.
In the rest of the text, washing by water return in a flow opposite the flow corresponding to the filtering phase will be called back-washing.
Other filtering apparatus comprise what is usually called a fixed floating bed. This floating bed is formed of materials whose mass per unit volume is less than that of the fluid to be treated which in general is water. This bed of materials which float in water is kept immersed by means of a known blocking device, such as a grid, and in this case we speak of a fixed floating bed.
Such a device is known from the British patent 1 305 399 in which the fixed floating bed is formed of granules or small balls of polystyrene held in the enclosure of a filter by a dividing wall perforated to dimensions less than the dimensions of the granules and disposed perpendicularly to the axis of symmetry of the enclosure of the filter. Filtering takes place in the depth of the layer of granules in a rising flow and the treated water is recovered in the top part of the filter, through an orifice situated close to its upper end whereas the untreated water is fed into the low part of the filter situated below the floating bed. Cleaning of the filter is achieved by backwashing by reversing the flow direction of the fluid through the filter and using the treated water flowing at a speed greater than the minimum fluidization speed so as to cause expansion of the bed and allow cleaning thereof.
This kind of apparatus for filtering in depth, which is the simplest and the most economic from the constructional point of view, has the drawback of requiring a large amount of treated water for backwashing. Since the amount of treated water required for cleaning the fluid is of the order of several times the capacity of the filter, this process results in considerable waste which is contradictory to the fact that by recycling and treating the water an attempt is made to economise it.
Another apparatus for filtering in depth and an associated method is known from the U.S. Pat. No. 4,547,286 which teaches the use of a floating bed formed by granules or small balls of floating materials such as polyethylene or polypropylene having a density close to that of water and of the order of 0.9. This document teaches as washing method the use of the agitated bed method by introducing air into the fluid so as to reduce the apparent density of the fluid formed by the liquid and the gas with respect to the granules of floating materials.
This method, economic from the water consumption point of view, requires the provision of a gas and a secondary fluid for varying the mean density of the fluid, which makes the construction and operation of such a filter more complex. Furthermore, this method limits the choice of filtering materials to those having a density less than 10 to 20% of that of the fluid to be treated.
It is also known to clean fixed floating bed filters in an upward flow by gravity backwashing. This method is taught by the U.S. Pat. No. 4,446,027 in which a floating bed of glass macrospheres of a mean diameter of 0.7 mm and having a coefficient of uniformity of 1.6 is fixed below a reservoir of purified fluid. This floating bed of small thickness, of the order of 0.3 m, is used for stopping at its surface the light materials which remain in suspension in the fluid following a first decantation phase. In this first phase, the heavy materials decant in a decantation zone of a lower reservoir situated below the floating bed. Furthermore, outside the decantation zone, the fluid in this lower reservoir is driven with a circulating movement causing agitation. The variable size of the balls forming the floating bed promotes the disposition of the largest balls in the upper part of the floating bed and of the smallest balls at the lower limit of the floating bed. This disposition thus reduces the possibility of .the materials in suspension penetrating into the filter.
Backwashing is carried out by using the filtered water from the upper reservoir which causes expansion of the floating bed and release of the materials in suspension which were fixed on the lower surface layer of the floating bed. This document teaches, during the filtering process, the combination of the phenomena of flocculation by agitation, decantation and additionally filtering by means of a rising flow in a fixed floating bed. Because of the low rate of surface clogging of the bed, backwashing by gravity requires little water.
The amount of water used for backwashing would certainly be very much higher in the case of cleaning a filter using a fixed floating bed as main means for filtering in depth. Thus, the U.S. Pat. No. 4,582,609 teaches the use of a fixed floating bed formed by an agglomerate of hollow glass microballs bonded by means of a resin and called glass macrospheres, polymer granules or small balls, such as epoxy resins or expanded materials such as polystyrene. Gravity backwashing of the filter is obtained by increasing the amount of filtered fluid contained above the bed, by immersing the floating bed mounted in a ballast device, then by driving filtered water through the fixed floating bed so as to create a fluidized bed. Driving of the filtered water is obtained by immersing the ballast-floating bed assembly. It is obvious, in this case, that the amount of treated fluid used for backwashing, is very great and much more than the percentage mentioned in the preceding patent. Furthermore, this type of device is complex and less economic to manufacture.